rndis_wlan: split add_wpa_key from rndis_iw_set_encode_ext

Split add_wpa_key() from rndis_iw_set_encode_ext so that conversion to cfg80211
would be easier later on.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>